B. Comparar consultas mediante EXISTS e IN

USE AdventureWorks ;
GO
SELECT a.FirstName, a.LastName
FROM Person.Contact AS a
WHERE EXISTS
(SELECT *
FROM HumanResources.Employee AS b
WHERE a.ContactId = b.ContactID
AND a.LastName = 'Johnson');
GO

En la siguiente consulta se utiliza IN.

USE AdventureWorks ;
GO
SELECT a.FirstName, a.LastName
FROM Person.Contact AS a
WHERE a.LastName IN
(SELECT a.LastName
FROM HumanResources.Employee AS b
WHERE a.ContactId = b.ContactID
AND a.LastName = 'Johnson');
GO